Mkdir: explicit umask and sequential tests

[shinhs0506]

1. explicitly set umask when testing file mode (mkdir: set correct permissions on dirs created by -p #4876 (comment))
2. run tests sequentially since tests that do set umask cause other tests to fail

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/timeout. tests/misc/timeout is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?

[cakebaker]

Why don't you set umask in test_symbolic_mode and test_multi_symbolic?

[cakebaker]

Never mind, you set the permissions in an "absolute" way.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?

[cakebaker]

Thanks!